The unfortunate truth is that the skin is here to stay, pretty much. It's not like a muscle where you can shorten the fibers through contraction. Skin doesn't work that way. If you were genetically gifted (like me, like many) with skin that doesn't have a lot of elastin in it, then I'd say you're stuck (like me, like many).
The name of the only "workout" I know of to get rid of the skin is called a tummy tuck...
Now, if the problem is just flabby muscles underneath, I recommend checking out or buying the book "Lose Your Mummy Tummy" by Julie Tupler. Her workout is a bit ridiculous & time consuming for most moms (i.e., those without nannies to watch their babies while they do their 500 repetitions of transversus abdominal pulls each morning...), but the explanations she provides of how/why your tummy muscles were extended during pregnancy and how/why you need to repair them are very well-written and easy to understand.
I've been a slacker on this subject myself.
Megan. Student Physical Therapist. Married & Mom to Geneva (6/30/06). Recent transplant to the rainy side of the Cascades.